Electric discharge devices of the mercury cathode type



y 1959 H. DE BOYNE KNIGHT 2,886,732

ELECTRIC DISCHARGE DEVICES OF THE MERCURY CATHODE TYPE Filed June 18, 1957 .lHendy de. Boyne Knight, Rugby,

ELECTRIC DISCHARGE DEVICES OF THE 'MERCURY CATHODE TYPE England, assignor to The British Thomson-Houston Company Limited, London, England, a British company Application June 18, 1957, Serial'No. 666,395

Claims priority, application 'Great Britain June 25, 1956 1 Claim. (CL. 313164) The invention relates to electric discharge devicesin whichcurrent passes as an arc discharge between an anode and a pool type cathode, preferably ofumercury, .the: discharge being initiated by..theg passage of current through the cathode from a stationary ignitor electrode.

In devices of the character in question, the level of the mercury in the cathode pool into which the ignitor electrode dips, may vary from time to time as a result of vaporisation of the mercury, during operation, or as the result of any tilting of the device as a whole. When the level varies, the ignition voltage required changes unless the ignitor electrode is designed to operate with varying levels and this renders sensitive operation difficult.

The object of the invention is to provide an arrangement in which the effective level of mercury adjacent the igniter electrode is maintained substantially constant, thereby reducing variation in ignition characteristics.

According to the invention the igniter electrode is located in the vicinity of a member constituted of a material wettable by the conducting liquid of which the pool is composed, in such a manner that the initiating arc from the igniter to the cathode pool passes between the igniter electrode and conducting liquid adherent to said member, and any gap containing vaporised pool material which may exist at the region of arc initiation.

The wettable member is hereinafter referred to as an anchoring device because of the tendency of the cathode spot associated with the are initiated through the igniter electrode to remain rooted on the surface of the anchoring device. The anchoring device may, and preferably does, project partially above the surface of the pool type cathode. It may be composed of an electrical conductor, for example, molybdenum, treated so as to be wetted by the conducting liquid, specifically mercury. The anchoring device may consist of metal sheet or wire, or of a composite structure of two or more laminae or wires, such that the conducting liquid wets the component parts and is held in the interstices between them. Alternatively, when the pool is of mercury, the anchoring device may consist of a porous body impregnated with the mercury, with or without any associated free mercury.

The invention will now be described with reference to the accompanying drawings, in which- Fig. 1 shows in cross section, an arc discharge device having a pool type cathode embodying the improvements of the invention;

Fig. 2 is a view of part of the members of Fig. 1, showing a modified construction of the igniter electrode and associated anchoring device, and

Figs. 3 and 4 are views similar ther alternative constructions.

Referring firstly to Fig. 1, the arc discharge device is seen to consist of a tubular envelope 1 of metal having flanged headers 2 and 3 inserted into the ends of the tubular portion of the envelope and arranged to support an anode 4 and a pool type cathode 5, respectively. The anode 4 is supported by the stem 6, insulatingly sealed to Fig. 2 showing furto the header 2 by wayJof an intermediate insulating mem- :ber 7, which .may, for :example,:be. of glass, to which are sealeda flanged member'8 connected to the anodelead 6, and:a second flanged member. 9 connected to .the 'rim .of an-aperture in the header 2. The inner flanged member .8 is connected to the anode lead 6 inta mannerxto make a hermetic seal thereto,: e.g. .by Welding, .andthe member 9 is similarly connected with the header 2. An

evacuatingtubulation 10is also provided on; the header 2,which.is sealed 01$, for example, by aiglass beam-11.

An igniter electrode 12 is supported from the header 3 in a manner to be insulated therefrom, the igniterelecitrode beingconnected by a transverse member13 with l a. terminal post 14 which. projects through the header :3, .being insulatedifrom the header andlfrom theppool 5 xby means of asurrounding glass tube 15.

.The header 3 supports the cathodepool 5=which prefwerablyrconsists .of mercury, thegigm'ter, electrode 12 dipping into the pool 5 so as to make contact therewith. It will be appreciated that any change in the level of the conducting liquid in the pool 5 caused by vaporisation of the liquid in the pool during operation, or by tilting of the device, will affect the distance into which the end of the igniter electrode 12 projects into the pool. This, in turn will aifect the ignition energy necessary to ignite an arc in the device. To overcome this disadvantage there is provided a cathode spot anchoring device 16 which consists of a solid member wettable by the mercury in the pool and located in such close proximity to the igniter electrode 12 that when an ignition potential is impressed on the igniter 12, the initiating arc passes between the igniter electrode and the cathode pool by way of mercury adhering to the anchoring device 16, and any gap containing ionised mercury which may be present there-between. The anchoring device may consist of a metal ring, as indicated in Fig. l, rigidly fixed to the header 3 and composed of a metal which is inherently wettable by mercury, or has been treated to cause it to be wetted by mercury, the anchoring device projecting above the level of the pool in the arrangement illustrated in Fig. 1.

When the arc has been started between the cathode 5 and the anode 4 by reason of an ignition potential applied to the igniter 12, the arc remains rooted on the anchoring device and consequently, the anchoring device should consist of a metal which will withstand the heat of the are rooted thereon. It has been found that molybdenum is a suitable metal to employ for the anchoring device, the molybdenum being treated to render it wettable by mercury by being cleaned electrolytically and fired in hydrogen and then subjected to positive ion bombardment in an arc in mercury vapour.

The header 3 is also provided with a terminal member 17 which forms the cathode connection for the device.

In an alternative arrangement shown in Fig. 2, the anchoring device 16 is constituted of a fiat plate 18 lying substantially parallel with the faces of the header 3 and supported by a vertical post 19, the plate 18 being located with its lower face substantially in alignment with the upper surface of the mercury level in the pool 5, the mercury spilling over the upper surface of the plate 18 which is of a material wet by mercury. The plate 18 may be provided with an aperture 20 into which the tip of the igniter 12 projects.

In the arrangement shown in Fig. 3, instead of providing the plate 18 with an aperture as indicated in Fig. 2, it is furnished with a depression 21 into which the tip of the igniter electrode 12 projects; the plate 18 may be supported by a metal stra as indicated.

In the arrangement shown in Figs. 1, 2 and 3, the igniter electrode is indicated as consisting of a material of semi-conductor type, e.g. known mixtures of boron carbide, boron nitride and boron. Instead of making the igniter electrode in this manner it may be composed of a heated member, for example, a wire 22, separately heated from a source23 through a transformer 24, as indicated in patent specification No. 2,070,501.

I It will be evident that by the use of the cathode spot anchoring device adjacent which the igniter electrode is located, variations in the level of mercury in the cathode pool will have little or no elfect on the ignition characteristics of the device. Furthermore, it is well-known that since the cathode spot is anchored to the anchoring device, little or no agitation of the surface of the mercury pool will arise, as would be the case if the cathode spot were allowed to wander over the surface of the mercury pool.

What I claim is:

In an electric discharge device comprising a sealed evacuated tubular metal envelope containing a mercury pool type cathode, an anode and a fixed igniter electrode dipping into said pool type cathode between which and said cathode an initiating arc is adapted to take place,

means for preventing effective variation in the level of a said conducting liquidadjacent said'igniter electrode coim prising an annular ring of a metal capable of being wet by said mercury immersed in said cathode pool and fixed to said envelope with one rim normally projecting above said pool, said rim being located so closely in the vicinity of said igniter electrode that'fsaid initiating arc, when formed, is thereby enabled to pass from said igniter electrode to said pool by way of mercury adherent to the rim of said ring projecting above said pool and any gap containing ionized mercury which is present therebetween. 0

References Cited in the file of this patent UNITED STATES PATENTS Re. 21,861 Miles July 22, 1941 2,070,501 Westendory Feb. 9, 1937 2,154,264 Damitz Apr. 11, 1939 2,155,138 Lerns Apr. 18, 1939 2,447,646 Halpine -Aug. 24,1948 2,743,387 Lewin et al. Apr. 24, 1956 2,776,392 Colaiaco Ian. 1,1957 

